Enhancements to a moderate pressure nitrogen and argon producing cryogenic air separation unit

ABSTRACT

Enhancements to the distillation column system and cycles for an argon and nitrogen producing cryogenic air separation unit are provided. The enhancements include systems and methods for: (i) recovery of xenon and krypton; (ii) production of oxygen product substantially free of hydrocarbons; and (iii) improvement in the design and performance of the super-stage argon column. The present systems and methods are further characterized in an oxygen enriched stream from the lower pressure column of the air separation unit is an oxygen enriched condensing medium used in the argon condenser.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. provisional patent application Ser. No. 63/025,347 filed May 15, 2020 the disclosure of which is incorporated by reference.

TECHNICAL FIELD

The present inventions relates to enhancements to a moderate pressure nitrogen and argon producing cryogenic air separation unit, and more particularly, to improvements to the distillation column system in the nitrogen and argon producing cryogenic air separation unit for (i) recovery of xenon and krypton; (ii) production of an oxygen product substantially free of hydrocarbons; and (iii) improvement in the design and performance of the super-stage argon column.

BACKGROUND

Industrial gas customers often seek argon and nitrogen product slates at volumes and pressures that are typically produced from a cryogenic air separation unit as disclosed in the technical publication Cheung, Moderate Pressure Cryogenic Air Separation Process, Gas Separation & Purification, Vol 5, March 1991 and U.S. Pat. No. 4,822,395 issued to Cheung. Similarly, U.S. patent application Ser. Nos. 15/962,205; 15/962,245; 15/962,292; and Ser. No. 15/962,358 filed on Apr. 25, 2018 as well as U.S. patent application Ser. No. 16/662,193 filed on Oct. 24, 2019, the disclosures of which are incorporated by reference herein, disclose new air separation systems and cycles that represent improvements over the system disclosed by Cheung. Such improvements to moderate pressure argon and nitrogen producing air separation units use an oxygen enriched stream taken from the lower pressure column as the condensing medium in the argon condenser to condense the argon-rich stream thus improving argon and nitrogen recoveries.

What is needed are further enhancements to the moderate pressure argon and nitrogen producing cryogenic air separation unit originally developed by Cheung and subsequently improved by the advancements disclosed in the above-identified U.S. patent application Ser. Nos. to expand the product slates or improve the performance and flexibility of such cryogenic air separation unit to meet customer requirements.

SUMMARY OF THE INVENTION

The present invention may be characterized as an air separation unit comprising: (i) a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; (ii) an adsorption based pre-purifier unit configured for removing water vapor, carbon dioxide, nitrous oxide, and hydrocarbons from the compressed air stream and producing a compressed and purified air stream; (iii) a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; (iv) a distillation column system having a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser-reboiler, the distillation column system further includes an argon column arrangement operatively coupled with the lower pressure column, the argon column arrangement having an argon column and an argon condenser, the distillation column system configured for receiving the cooled, compressed and purified air stream and produce one or more oxygen enriched streams from the lower pressure column; an argon product stream, a gaseous nitrogen product stream; and (v) wherein the distillation column system further comprises a rare gas rectification system configured to receive an argon-oxygen enriched stream from the lower pressure column and to produce a crude rare gas stream. At least one of the oxygen enriched streams from the lower pressure column is an oxygen product stream and at least one of the oxygen enriched streams from the lower pressure column is an oxygen enriched condensing medium directed to the argon condenser. The rare gas rectification system preferably has between 2 and 6 theoretical stages of separation and is configured to recover between 90% to 99% of the xenon and between about 10% to 90% of the krypton in the compressed and purified air stream.

In one embodiment, the rare gas rectification system further comprises an additional separation section of trays disposed in the lower pressure column just above the condenser-reboiler and the crude rare gas stream is extracted from the bottom of the lower pressure column whereas I another embodiment the rare gas rectification system further comprises an additional separation section of trays disposed in the argon condenser vessel just above the argon condenser and the crude rare gas stream is extracted from the bottom of the argon condenser vessel.

In still other embodiments of the air separation unit the rare gas rectification system further comprises a bifurcated separation section of trays with a first part of the separation section disposed in an argon condenser vessel just above the argon condenser and a second part of the of the separation section disposed either in in the lower pressure column just above the condenser-reboiler or in a stand-alone rare gas column.

The present invention may also be characterized as an air separation unit comprising: (i) a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; (ii) an adsorption based pre-purifier unit configured for removing water vapor, carbon dioxide, nitrous oxide, and hydrocarbons from the compressed air stream and producing a compressed and purified air stream; (iii) a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; and (iv) a distillation column system having a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser-reboiler, the distillation column system further includes an argon column arrangement operatively coupled with the lower pressure column, the argon column arrangement having an argon column, an argon condenser, and a reboiler. The distillation column system is configured for receiving the cooled, compressed and purified air stream and produce a first oxygen enriched stream from the bottom of the lower pressure column; an argon product stream, a gaseous nitrogen product stream wherein all or a portion of the oxygen enriched streams from the bottom of lower pressure column is directed to the argon condenser to be used as an oxygen enriched condensing medium. The argon condenser is configured to condense the argon-enriched overhead against the oxygen enriched condensing medium to produce a crude argon stream, an argon reflux stream, an oxygen enriched waste stream, and optionally an oxygen product stream.

The argon column arrangement is configured to receive an argon-oxygen enriched stream from the lower pressure column at the reboiler and produce a condensed argon-oxygen enriched stream that is let down in pressure and directed to an intermediate location of the argon column. The argon column arrangement is further configured to produce another oxygen enriched stream that is returned to or released into the lower pressure column and an argon-enriched overhead that is directed to the argon condenser.

Lastly, the present invention may also be characterized as an air separation unit comprising: (i) a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; (ii) an adsorption based pre-purifier unit configured for removing water vapor, carbon dioxide, nitrous oxide, and hydrocarbons from the compressed air stream and producing a compressed and purified air stream; (iii) a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; (iv) a distillation column system having a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser-reboiler, the distillation column system further includes an argon column arrangement operatively coupled with the lower pressure column, the argon column arrangement having an argon column and an argon condenser, the distillation column system configured for receiving the cooled, compressed and purified air stream and a first oxygen enriched stream from the lower pressure column to be used as the condensing medium in the argon condenser; an argon product stream, a gaseous nitrogen product stream; and (v) a supplemental oxygen column. The argon column arrangement is configured to receive an argon-oxygen enriched stream from the lower pressure column and produce a second oxygen enriched stream that is returned to or released into the lower pressure column and an argon-enriched overhead that is directed to the argon condenser. The supplemental oxygen column is configured to receive an oxygen enriched stream from the argon column, either as a portion of the oxygen enriched stream to be returned to the lower pressure column or as a separate oxygen enriched stream taken from an intermediate location of the argon column. The supplemental oxygen column is configured to rectify the received oxygen enriched stream to produce an oxygen enriched overhead stream that is returned to the argon column and a hydrocarbon-free oxygen liquid stream. The supplemental oxygen column includes a reboiler disposed proximate the bottom of the supplemental oxygen column and is configured to boil oxygen in the supplemental oxygen column against a stream of nitrogen received from the higher pressure column or a portion of the compressed and purified air stream to produce an ascending oxygen vapor in the supplemental oxygen column and a condensed nitrogen stream.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic process flow diagram of a cryogenic air separation unit capable of operating at moderate pressure and having high nitrogen recovery and high argon recovery;

FIG. 2 is a schematic process flow diagram of an alternate embodiment of a cryogenic air separation unit capable of operating at moderate pressure and having high nitrogen recovery and high argon recovery;

FIG. 3 is a schematic process flow diagram of a yet another embodiment of a cryogenic air separation unit capable of operating at moderate pressure and having high nitrogen recovery and high argon recovery;

FIG. 4 is a schematic process flow diagram of still another embodiment of a cryogenic air separation unit capable of operating at moderate pressure and having high nitrogen recovery and high argon recovery;

FIG. 5 is a partial schematic diagram of a modified lower pressure column of the distillation column system of FIGS. 1-4 suitable for the recovery of xenon and krypton in accordance with an aspect or embodiment of the present invention;

FIG. 6 is a partial schematic diagram of a modified argon condenser vessel of the distillation column system of FIGS. 1-4 suitable for the recovery of xenon and krypton in accordance with another aspect or embodiment of the present invention;

FIG. 7 is a partial schematic diagram of a modified lower pressure column and modified argon condenser vessel of the distillation column system shown in FIGS. 1-4 that is suitable for the recovery of xenon and krypton in accordance with yet another aspect or embodiment of the present invention;

FIG. 8 is another partial schematic diagram of a modified lower pressure column and modified argon condenser vessel of the distillation column system shown in FIGS. 1-4 that is suitable for the recovery of xenon and krypton in accordance with still another aspect or embodiment of the present invention;

FIGS. 9A and 9B depict graphs showing the expected xenon and krypton recoveries as a function of argon condenser recirculation for the air separation unit of FIGS. 1-4 with the modified distillation column systems;

FIG. 10 is a partial schematic diagram of a modified lower pressure column, higher pressure column and argon column arrangement of the distillation column system of FIGS. 1-4 suitable for the recovery of an oxygen product substantially free of hydrocarbon impurities;

FIG. 11 is a partial schematic diagram of a modified column arrangement of the distillation column system of FIGS. 1-4 suitable for the recovery of an oxygen product substantially free of hydrocarbon impurities; and

FIG. 12 is a schematic diagram of a of a cryogenic air separation unit similar to those shown in FIGS. 1-4 but with a modified distillation column system suitable for the enhanced design and performance of the super-staged argon column.

DETAILED DESCRIPTION

The presently disclosed systems and methods provides for enhancements to the distillation column system in a moderate pressure nitrogen and argon producing cryogenic air separation unit, including (i) enhancements for the recovery of xenon and krypton; (ii) enhancements for the production of a hydrocarbon free oxygen product; (iii) enhancements to the argon super-stage column; and (iv) enhancements to the air separation cycles. Each of these enhancements together with the baseline moderate pressure nitrogen and argon producing cryogenic air separation unit will be described in the sections that follow.

Moderate Pressure Argon and Nitrogen Producing Cryogenic Air Separation Unit

As discussed in more detail below, the disclosed moderate pressure argon and nitrogen producing cryogenic air separation unit comprises a multi-column arrangement and achieves the high argon and nitrogen recoveries by using a portion of an oxygen enriched stream taken from the lower pressure column as the condensing medium in the argon condenser to condense the argon-rich stream. The oxygen rich boil-off from the argon condenser is then used as a purge gas to regenerate the adsorbent beds in the adsorption based pre-purifier unit. The disclosed cryogenic air separation system and methods are further capable of limited oxygen production.

Turning to FIG. 1, there is shown a schematic illustration of the baseline argon and nitrogen producing cryogenic air separation unit 10 having high nitrogen and argon recoveries. In a broad sense, the depicted air separation unit includes a main feed air compression train or system, a turbine air circuit, an optional booster air circuit, a primary heat exchanger system, and a distillation column system. As used herein, the main feed air compression train, the turbine air circuit, and the booster air circuit, collectively comprise the ‘warm-end’ air compression circuit. Similarly, main heat exchanger, portions of the turbine based refrigeration circuit and portions of distillation column system are referred to as ‘cold-end’ equipment that are typically housed in insulated cold boxes.

In the main feed compression train shown in FIG. 1 the incoming feed air 22 is typically drawn through an air suction filter house and is compressed in a multi-stage, intercooled main air compressor arrangement 24 to a pressure that can be between about 6.5 bar(a) and about 11 bar(a). This main air compressor arrangement 24 may include integrally geared compressor stages or a direct drive compressor stages, arranged in series or in parallel. The compressed air stream 26 exiting the main air compressor arrangement 24 is fed to an aftercooler (not shown) with integral demister to remove the free moisture in the incoming feed air stream. The heat of compression from the final stages of compression for the main air compressor arrangement 24 is removed in aftercoolers by cooling the compressed feed air with cooling tower water. The condensate from this aftercooler as well as some of the intercoolers in the main air compression arrangement 24 is preferably piped to a condensate tank and used to supply water to other portions of the air separation plant.

The cool, dry compressed air stream 26 is then purified in a pre-purification unit 28 to remove high boiling contaminants from the cool, dry compressed air feed. A pre-purification unit 28, as is well known in the art, typically contains two beds of alumina and/or molecular sieve operating in accordance with a temperature swing adsorption cycle in which moisture and other impurities, such as carbon dioxide, water vapor and hydrocarbons, are adsorbed. While one of the beds is used for pre-purification of the cool, dry compressed air feed while the other bed is regenerated, preferably with a portion of the waste nitrogen from the air separation unit. The two beds switch service periodically. Particulates are removed from the compressed, pre-purified feed air in a dust filter disposed downstream of the pre-purification unit 28 to produce the compressed, purified air stream 29.

The compressed and purified air stream 29 is separated into oxygen-rich, nitrogen-rich, and argon-rich fractions in a plurality of distillation columns including at least a higher pressure column 72, a lower pressure column 74, and an argon column 129. Prior to such distillation however, the compressed and pre-purified air stream 29 is typically split into a plurality of feed air streams, which may include a boiler air stream 32, a turbine air stream 31, and a non-boosted air stream 33.

As shown in FIG. 1, the boiler air stream 32 may be further compressed in a booster compressor 34 and subsequently cooled in an aftercooler 39 to form a boosted pressure air stream 36 which is then further cooled in the main heat exchanger 52. Likewise, turbine air stream 31 may be further compressed in a turbine air booster compressor 37 and subsequently cooled in an aftercooler 39 to form a boosted pressure turbine air stream 38 which is then further cooled in the main heat exchanger 52.

Cooling the booster air stream 36 and non-booster air stream 33 and partially cooling the boosted turbine air stream 38 in the main heat exchanger 52 is preferably accomplished by way of indirect heat exchange with the warming streams which include the oxygen streams 197, 386 as well as nitrogen streams 195 from the distillation column system to produce cooled air streams at temperatures suitable for rectification in the distillation column systems.

The partially cooled turbine air stream 38 is expanded in turbine 35 to produce exhaust stream 64 that is directed to the lower pressure column 74. In this manner, a portion of the refrigeration for the air separation unit 10 is thus provided by the expansion of the turbine air stream 38 in turbine 35. The boosted pressure air stream is fully cooled and exits the cold end of main heat exchanger 52 as elevated pressure air stream 48. The non-boosted air stream 31 is also fully cooled in the main heat exchanger 52 to produce a fully cooled air stream 47. The fully cooled non-boosted air stream 47 as well as the elevated pressure air stream 48 are introduced into higher pressure column 72. Preferably, the boosted pressure stream 48 is preferably let down in pressure in valve 49 and fed to higher pressure column 72 at a location several stages above the bottom where the non-boosted air stream 47 is introduced into the higher pressure column 72.

The main heat exchanger 52 is preferably a brazed aluminum plate-fin type heat exchanger. Such heat exchangers are advantageous due to their compact design, high heat transfer rates and their ability to process multiple streams. They are manufactured as fully brazed and welded pressure vessels. For small air separation unit units, a heat exchanger comprising a single core may be sufficient. For larger air separation unit units handling higher flows, the heat exchanger may be constructed from several cores which must be connected in parallel or series.

Alternate arrangements for processing the air streams that do not include booster compressor 34 are shown in FIGS. 2-4. As seen in FIG. 4, the turbine air stream 31 and boiler air stream 32 remain combined are further compressed in compressor 37, cooled in aftercooler 39 and then split into the boosted pressure air stream 36 and the booster turbine air stream 38 both of which are directed to separate heat exchange passages within the main heat exchanger 52. The boosted turbine air stream 38 is partially cooled and extracted from an intermediate location of the main heat exchanger 52 and then expanded in turbine 35 to produce exhaust stream 64 that is directed to the lower pressure column 74. The boosted air stream 36 as well as the non-boosted air stream 33 are fully cooled in the main heat exchanger 52 and then directed to the higher pressure column 74 as described above with reference to FIG. 1.

In the embodiment illustrated in FIG. 3, the turbine air stream 31 and boiler air stream 32 also remain combined are further compressed in compressor 37, cooled in aftercooler 39 and then directed to a common heat exchange passage within the main heat exchanger 52. The stream is then split within the main heat exchanger such that the boosted turbine air stream 38 is partially cooled and extracted from an intermediate location of the main heat exchanger 52 while the boosted air stream 36 continues through the main heat exchanger where it is fully cooled and directed to the higher pressure column 74 as described above with reference to FIG. 1. The partially cooled turbine air stream 38 is expanded in turbine 35 to produce exhaust stream 64 that is directed to the lower pressure column 74. The non-boosted air stream 33 is fully cooled in a separate heat exchange passage of the main heat exchanger 52 and then also directed to the higher pressure column 74 as described above.

The embodiment of FIG. 2 shows a further arrangement where the turbine air stream 31 is not boosted, so there is no need for the turbine air compressor. In this embodiment, the boiler air stream 32 is further compressed in booster compressor 34 and cooled in aftercooler 39 and directed to main heat exchanger 52 where it is fully cooled and then directed to the higher pressure column 74 as described above with reference to FIG. 1. Similarly, the non-booster air stream 33 is also directed to main heat exchanger 52 where it is fully cooled and then directed to the higher pressure column 74 as described above with reference to FIG. 1. The non-boosted turbine air stream 38 is partially cooled and extracted from an intermediate location of the main heat exchanger 52 and then expanded in turbine 35 to produce exhaust stream 64 that is directed to the lower pressure column 74.

The turbine based refrigeration circuits used in cryogenic air separation units are often referred to as either a lower column turbine (LCT) arrangement or an upper column turbine (UCT) arrangement which are used to provide refrigeration to a two-column or three column cryogenic air distillation column systems. In the UCT arrangements shown in FIGS. 1-3, the boosted turbine air stream is preferably at a pressure in the range from between about 6 bar(a) to about 10.7 bar(a) and partially cooled to a temperature in a range of between about 140 K and about 220 K. This cooled, compressed turbine air stream that is introduced into the turbine to produce a cold exhaust stream 64 that is then introduced into the lower pressure column of the distillation column system. The supplemental refrigeration created by the expansion of the turbine air stream is thus imparted directly to the lower pressure column thereby alleviating some of the cooling duty of the main heat exchanger. In some embodiments, the turbine may be coupled with a compressor, either directly or by appropriate gearing.

While the turbine based refrigeration circuit illustrated in the FIGS. 1-4 is shown as an upper column turbine (UCT) circuit where the turbine exhaust stream is directed to the lower pressure column, it is contemplated that the turbine based refrigeration circuit alternatively may be a lower column turbine (LCT) circuit or a partial lower column (PLCT) where the expanded exhaust stream is fed to the higher pressure column of the distillation column system. Still further, turbine based refrigeration circuits may be some variant or combination of LCT arrangement, UCT arrangement and/or a warm recycle turbine (WRT) arrangement, generally known to those persons skilled in the art.

The aforementioned components of the incoming feed air stream, namely oxygen, nitrogen, and argon are separated within the distillation column system that includes a higher pressure column 72, a lower pressure column 74, an argon column 129, a condenser-reboiler 75 and an argon condenser 78. The higher pressure column 72 typically operates in the range from between about 6 bar(a) to about 10 bar(a) whereas lower pressure column 74 operates at pressures between about 1.5 bar(a) to about 2.8 bar(a). The higher pressure column 72 and the lower pressure column 74 are preferably linked in a heat transfer relationship such that all or a portion of the nitrogen-rich vapor column overhead, extracted from proximate the top of higher pressure column 72 as stream 73, is condensed within a condenser-reboiler 75 located in the base of lower pressure column 74 against the oxygen-rich liquid column bottoms 77 residing in the bottom of the lower pressure column 74. The boiling of oxygen-rich liquid column bottoms 77 initiates the formation of an ascending vapor phase within lower pressure column 74. The condensation produces a liquid nitrogen containing stream 81 that is divided into a clean shelf reflux stream 83 that may be used to reflux the lower pressure column 74 to initiate the formation of descending liquid phase therein and a nitrogen-rich stream 85 that is used as reflux to the higher pressure column 72.

Cooled feed air stream 47 is preferably a vapor air stream slightly above its dew point, although it may be at or slightly below its dew point, that is fed into the higher pressure column for rectification resulting from mass transfer between an ascending vapor phase and a descending liquid phase that is initiated by a nitrogen based reflux stream 85. The mass transfer occurs within a plurality of mass transfer contacting elements, illustrated as distillation trays 71. This produces crude liquid oxygen column bottoms 86, also known as kettle liquid which is taken as stream 88, and the nitrogen-rich column overhead 89, taken as clean shelf liquid stream 83.

In the lower pressure column, the ascending vapor phase includes the boil-off from the condenser-reboiler as well as the exhaust stream 64 from the turbine 35 which is subcooled in subcooling unit 99B and introduced as a vapor stream at an intermediate location of the lower pressure column 72. The descending liquid is initiated by nitrogen reflux stream 83, which is sent to subcooling unit 99A, where it is subcooled and subsequently expanded in valve 96 prior to introduction to the lower pressure column 74 at a location proximate the top of the lower pressure column.

Lower pressure column 74 is also provided with a plurality of mass transfer contacting elements, that can be trays or structured packing or other known elements in the art of cryogenic air separation. The contacting elements in the lower pressure column 74 are illustrated as structured packing 79. The separation occurring within lower pressure column 74 produces an oxygen-rich liquid column bottoms 77 extracted as a high purity oxygen enriched liquid stream 377 having an oxygen concentration of greater than 99.5%. In addition, a lower purity oxygen enriched stream 90 is also extracted from the lower pressure column several stages above the condenser 75 as the condensing medium to condense the argon-rich stream. The lower pressure column further produces a nitrogen-rich vapor column overhead that is extracted as a gaseous nitrogen product stream 95. If needed, a small portion of the subcooled nitrogen reflux stream 83 may be taken via valve 101 as liquid nitrogen product 98.

The high purity oxygen enriched liquid stream 377 can be separated into a first oxygen enriched liquid stream 380 that is pumped in pump 385 and the resulting pumped oxygen stream 386 is directed to the main heat exchanger 52 where it is warmed to produce a high purity gaseous oxygen product stream 390. A second portion of the high purity oxygen enriched liquid stream 377 may be taken as a liquid oxygen product 185. The lower purity oxygen enriched liquid stream 90 is preferably subcooled in subcooling unit 99B via indirect heat exchange with the oxygen enriched waste stream 196 and then pumped via pump 180 to argon condenser 78 where it is used to condense argon-rich stream 126 taken from the overhead 123 of argon column.

The vaporized oxygen stream that is boiled off from the argon condenser 78 is an oxygen enriched waste stream 196 that is warmed within subcooler 99B. The warmed oxygen enriched waste stream 197 is directed to the main or primary heat exchanger and then used as a purge gas to regenerate the adsorption based prepurifier unit 28. Additionally, a waste nitrogen stream 93 may be extracted from the lower pressure column to control the purity of the gaseous nitrogen product stream 95. The waste nitrogen stream 93 is preferably combined with the oxygen enriched waste stream 196 upstream of subcooler 99B. Also, vapor waste oxygen stream 97 may be needed in some cases when more oxygen is available than is needed to operate argon condenser 78, typically when argon production is reduced.

A liquid stream is withdrawn from argon condenser vessel, may be passed through gel trap and returned to the base or near the base of lower pressure column. The gel trap serves to remove carbon dioxide, nitrous oxide, and certain heavy hydrocarbons that might otherwise accumulate in the system. Alternatively, a small flow can be withdrawn via stream as a drain from the system such that gel trap is eliminated.

Conventionally, the argon condenser shown in FIGS. 1-4 is of a pool boiler (i.e. thermosyphon) design. Alternatively, the argon condenser can be of a once through upflow design. These are well known. Optionally, the argon condenser shown in FIGS. 1-4 is a downflow argon condenser. The downflow configuration makes the effective delta temperature (ΔT) between the condensing stream and the boiling stream smaller. As indicated above, the smaller ΔT may result in reduced operating pressures within the argon column, lower pressure column, and higher pressure column, which translates to a reduction in power required to produce the various product streams as well as improved argon recovery. The use of the downflow argon condenser also enables a potential reduction in the number of column stages, particularly for the argon column. Use of an argon downflow condenser is also benefitted from a capital standpoint, in part, because pump 180 is already required in the presently disclosed air separation cycles. Also, since liquid stream 130 already provides a continuous liquid stream exiting the argon condenser shell which also provides the necessary wetting of the reboiling surfaces to prevent the argon condenser from ‘boiling to dryness’ and maintain safe operation.

Nitrogen product stream 95 is passed through subcooling unit 99A to subcool the nitrogen reflux stream 83 and kettle liquid stream 88 via indirect heat exchange. As indicated above, the subcooled nitrogen reflux stream 83 is expanded in valve 96 and introduced into an uppermost location of the lower pressure column 74 while the subcooled the kettle liquid stream 88 is expanded in valve 107 and introduced to an intermediate location of the lower pressure column 74. After passage through subcooling units 99A, the warmed nitrogen stream 195 is further warmed within main heat exchanger 52 to produce a warmed gaseous nitrogen product stream 295.

The flow of the first oxygen enriched liquid stream 380 may be up to about 20% of the total oxygen enriched streams exiting the system. The argon recovery of this arrangement is between about 75% and 96% or higher which is greater than the prior art moderate pressure air separation systems. Although not shown, a stream of liquid nitrogen taken from a nitrogen liquefier or from an external source may be combined with the oxygen enriched liquid stream 90 to condense the argon-rich stream 126 in the argon condenser 78, to enhance argon recovery.

With liquid nitrogen add, the boiling refrigerant in the argon condenser is a mix of liquid oxygen and liquid nitrogen and will be generally colder than the boiling refrigerant disclosed in U.S. patent application Ser. Nos. referenced above. As a result, the distillation column system pressures may be naturally lower. In other words, the cryogenic air separation unit, and specifically the compressors and distillation column system, may be designed to take advantage of this lower operating pressure which would result in an overall power savings. Alternatively, if it is not feasible or desirable to design the compressors and distillation columns of cryogenic air separation unit for the required pressure ranges, the vaporized waste gas from the argon condenser may be back pressured at the warm end of the main heat exchanger. By doing this back pressuring in combination with liquid nitrogen add, the boiling fluid temperature in the argon condenser is not altered and the distillation column system pressures will also remain the same. Employing this alternate back pressuring method would be the likely method of operation if the higher liquid oxygen production is expected to be infrequent or non-continuous.

Recovery of Xenon and Krypton

Turning now to FIGS. 5-8 there are shown partial schematic diagrams of selected portions of the distillation column system. Many of the features, components and streams associated with the lower pressure column arrangement shown in FIGS. 5-8 are similar or identical to those described above with reference to FIGS. 1-4 and for sake of brevity will not be repeated here.

The key difference between the lower pressure column arrangement illustrated in FIG. 5 compared to the lower pressure column arrangements shown in FIGS. 1-4 is the inclusion of an additional separation section 502 at the bottom region of the lower pressure column 74 just above the condenser-reboiler 75. The additional separation section 502 preferably consists of between 2 and 6 theoretical stages of separation, which could be in the form of 2 to 6 trays or an equivalent height of structured packing elements.

The oxygen enriched stream 90 is extracted from the lower pressure column 74 several stages above the additional separation section 502 and directed to the argon condenser 78 to be used as the condensing medium to condense the argon-rich stream. The product oxygen stream 377, if any, is also preferably withdrawn from the lower pressure column 74 at a location just above the additional separation section 502 while a crude rare gas liquid stream 510 is extracted from the bottom section or base of lower pressure column 74.

Turning now to FIG. 6, the difference between the distillation column system arrangement shown in FIG. 6 compared to the distillation column system arrangements shown in FIGS. 1-4 is the inclusion of an additional separation section 504 in the argon condenser vessel 120 at the upper region of the vessel just above the argon condenser 78. The additional separation section 504 in the argon condenser vessel 120 also preferably consists of between 2 and 6 theoretical stages of separation, which could be in the form of 2 to 6 trays. The differences further include the extraction of the crude rare gas liquid stream 510 from the bottom section or base of the argon condenser vessel 120 and extraction of a recirculation liquid stream 515 from an intermediate location of the argon condenser vessel 120 that, if needed, is returned to an intermediate location of the lower pressure column 74.

Turning now to FIGS. 7-8, the notable differences between the illustrated distillation column system arrangements compared to the distillation column system arrangements shown in FIGS. 1-4 are the inclusion of a split or bifurcated separation section 506A and 506B to facilitate the recovery of xenon and krypton. A first part of the bifurcated separation section 506A is disposed in the argon condenser vessel 120 at the uppermost region of the vessel just above the argon condenser 78 while the second part of the bifurcated separation section 506B is disposed in the bottom region of the lower pressure column 74 just above the condenser-reboiler 75. Collectively, the bifurcated separation section 506A, 506B preferably consists of between 2 and 6 theoretical stages of separation.

In the embodiment depicted in FIG. 7, the oxygen enriched liquid stream 190 containing rare gas is extracted from the lower pressure column 74 at a location just above the second part of the bifurcated separation section 506B and fed into the argon condenser vessel 120. Pumping and subcooling of analogous stream 90, as shown in FIGS. 1-4 is employed for stream 190, but not shown for simplicity. The fed stream undergoes some further separation in the first part of the bifurcated separation section 506A with the resulting oxygen enriched waste gas 196 removed from the overhead of the argon condensing vessel 120 and the descending liquid used as the condensing medium to the argon condenser with the bottoms liquid extracted as a rare gas containing liquid stream 508 which is returned to the lower pressure column 74. The rare gas containing liquid stream 508 is fed into the lower pressure column 74 at a location just above the second part of the bifurcated separation section 506B where it undergoes further separation while the crude rare gas stream 510 is extracted from the bottom section or base of lower pressure column 74. This arrangement improves the potential recovery of krypton that would otherwise be lost in the oxygen enriched waste stream leaving the argon condenser.

The embodiment depicted in FIG. 8 is similar to the embodiment in FIG. 7 except that the part of the bifurcated separation section that was incorporated into the lower pressure column is now embodied as a separation section 506C in separate column 570. The separate column 570 preferably includes several xenon and krypton separation stages 506 as well as a reboiler 525 that receives a liquid nitrogen stream 526 from the higher pressure column and returns the vaporized nitrogen stream 528 to the higher pressure column. The entire liquid feed stream can be supplied via stream 511. Preferably, though, about 25% of the feed is supplied via stream 511 and 75% is supplied via stream 512. This enables considerably increased Kr and Xe enrichment before the constraining concentrations of other contaminants are reached. Separate column 570 enables this, as this result is entirely dependent on reducing the internal reflux ratio in the rare gas separation. The previously described embodiments do not have this capability because the influence of the rare gas feed has only a very small effect on the internal reflux ratio when the rare gas separation is part of the low pressure column. Furthermore, separate column 570 is preferably smaller in diameter than the lower pressure column 74 and is advantageous in that it reduces the height of the lower pressure column compared to the previously described embodiments. The separate column 570 also may reduce the capital costs associated with the structured packing or large trays used in the additional stages of separation in the embodiment in FIG. 7 as it is likely less costly to add stages of separation in the small diameter column 570 to improve krypton recovery. By moving the rare gas enrichment stages 506C out of lower pressure column 74 a small reduction in compression power is also realized.

As depicted in FIGS. 9A and 9B, xenon recovery of between about 90% to 99% or more and krypton recovery of between about 10% to 90% or more was shown for the contemplated distillation column system modifications. The expected xenon and krypton recoveries are based on computer simulations of the present modifications to the distillation column system and very much depend on how many stages of separation are included in the additional separation section how much oxygen is recirculated back to the lower pressure column from the argon condenser. Without being bound by any particular theory, the high recoveries of xenon and krypton using the present modifications to the distillation column system is likely due to the back pressure of the air separation cycle which has a tendency to keep the heavy components in the liquid phase.

Production of a Hydrocarbon Free Oxygen Stream

Turning now to FIGS. 10-11, there are shown partial schematic diagrams of selected portions of the modified distillation column system. Many of the features, components and streams associated with the lower pressure column arrangement shown in FIG. 10 and FIG. 11 are similar or identical to those described above with reference to FIGS. 1-4 and for sake of brevity will not be repeated here.

The key difference between the modified distillation column systems shown in FIGS. 10-11 compared to the distillation column systems shown in FIGS. 1-4 is the inclusion of a supplemental oxygen column 600. The supplemental oxygen column 600 is configured to produce a liquid oxygen product 610 substantially free of hydrocarbons, preferably less than about 50 ppm hydrocarbon impurities and more preferably less than or equal to about 10 ppm hydrocarbon impurities.

Hydrocarbon impurities are generally introduced to the lower pressure column of the distillation column system primarily through the kettle liquid feed to the lower pressure column. All of the hydrocarbons work their way down the lower pressure column with the descending liquid where the hydrocarbons are concentrated. The ascending vapor is in equilibrium with the descending liquid but contains much lower concentrations of hydrocarbon impurities because of the high relative volatilities. In fact, the ascending vapor in the lower pressure column at the argon column takeoff point generally contains only about 1 ppb of the heavy hydrocarbons (i.e. heavier than methane) and about 4 ppm methane or other light hydrocarbons. Given the low content of hydrocarbon impurities in the ascending vapor in the lower pressure column, the vapor feed from the intermediate location of lower pressure column to the argon column also contains the same hydrocarbon impurity content as the ascending vapor in the lower pressure column. Consequently, the liquid streams in the argon column as well as the liquid stream returned to the lower pressure column from the argon column are also very low in hydrocarbon impurities.

In the embodiment illustrated in FIG. 10, the supplemental oxygen column 600 is configured to receive a liquid stream 602, enriched in oxygen, from the argon column 129 and rectify the received oxygen enriched stream 602 to produce an oxygen enriched overhead vapor stream 604 that is returned to the argon column and a hydrocarbon-free oxygen liquid stream 610. The illustrated supplemental oxygen column preferably has about 32 theoretical stages of separation and is configured as a re-boiled oxygen column. The reboiler 615 is disposed proximate the bottom of the supplemental oxygen column and is configured to boil oxygen against a stream of nitrogen 606 received from the higher pressure column to produce an ascending oxygen vapor in the supplemental oxygen column 600 and a condensed nitrogen stream 608. Condensed nitrogen stream 608 is returned to the high pressure column. Alternatively, a diverted stream of compressed and purified air may be used in lieu of the nitrogen streams.

In the embodiment illustrated in FIG. 11, the supplemental oxygen column 600 is configured to receive a liquid stream 612, enriched in oxygen, from an intermediate location of the argon column 129 and rectify the received oxygen enriched stream 612 to produce an oxygen enriched overhead vapor stream 614 that is returned to the argon column and a hydrocarbon-free oxygen liquid stream 610. In the illustrated embodiment, oxygen enriched stream 612 is taken from a location about 8 stages from the bottom of the argon column 129 and the oxygen enriched overhead vapor stream 614 is returned to an intermediate location of the argon column, preferably the same location or just below the extraction point of the oxygen enriched stream 612. As with the embodiment of FIG. 10, this embodiment also has about 32 stages of separation and includes reboiler 615 is disposed proximate the bottom of the supplemental oxygen column and configured to boil oxygen against a stream of nitrogen 606 received from the higher pressure column to produce an ascending oxygen vapor in the supplemental oxygen column 600 and a condensed nitrogen stream 608. As was the case for FIG. 10, alternatively a diverted stream of compressed and purified air may be used in lieu of the nitrogen streams.

In lieu of the separate stand-alone supplemental oxygen column vessel as illustrated in FIGS. 10 and 11, contemplated alternative arrangements include disposing or integrating the supplemental oxygen column within the lower pressure column shell, perhaps as a divided wall type column configuration such as an annular divided column annular wall arrangement or a split divided column annular wall arrangement in the lower portions of the column shell. Alternatively, the supplemental oxygen column may be combined or integrated with the argon column shell.

Super-Stage Argon Column with Reboiler

Turning now to FIG. 12 there is shown a schematic diagram of a cryogenic air separation unit similar to those shown in FIGS. 1-4 but with a modified distillation column system. Many of the features, components and streams associated with the distillation column system arrangement shown in FIG. 12 are similar or identical to those described above with reference to FIGS. 1-4 and for sake of brevity will not be repeated here.

The key differences between the distillation column system arrangement for a cryogenic argon and nitrogen producing air separation unit shown in FIG. 12 compared to the lower pressure column arrangements shown in FIGS. 1-4 are found in the super-stage argon column arrangement where the argon rectifier produces merchant grade argon directly from the columns. Specifically, the key difference is the use of a reboiler 199 positioned at the base of the argon super-stage column 129 to condense the argon column feed stream. U.S. Pat. No. 5,305,611, the disclosure of which is incorporated by reference herein, provides a similar super-stage reboiling concept for an oxygen producing plant that uses a kettle stream from the higher pressure column as the condensing medium in the argon condenser in lieu of the oxygen enriched stream from the lower pressure column.

The feed stream to the argon column 121 taken from the lower pressure column 74 is substantially condensed within a reboiler 200 positioned at the base of the argon column 129, as shown in the embodiment of FIG. 12. The condensed stream 201 exiting the reboiler 200 is then depressurized in valve 198 and fed as stream 199 to an intermediate location of the argon column 129. In so doing, the relative recovery of the argon column is increased. In other words, a greater fraction of the argon contained in the feed stream that is fed to the argon column is obtained as argon product. Use of the reboiler at the base of the argon super-stage column, reduces the net volume occupied by the argon column. While the illustrated embodiment shows all of the condensed feed stream being fed to the intermediate location of the argon column, it is contemplated that a portion of this condensed feed stream may be returned directly to the lower pressure column.

Mechanical pump 180 may be used to motivate the oxygen liquid from the lower pressure column while pump 188 may be used to motivate the argon depleted bottoms from the argon column to/from the other columns. The argon depleted bottoms 123 of the argon column may be directed via a pump to a location in the lower pressure column below the argon column feed draw point. Although not shown, pumps may also be used to split the argon column into two or more sections as necessary to reduce the overall physical height of the argon super-stage column. FIG. 12 also contemplates the use of a waste nitrogen expansion cycle including the expansion of a partially cooled waste nitrogen stream in turboexpander 193 and the resulting expanded stream 194 cooled in a separate passage of the main heat exchanger 52 to yield the warmed waste nitrogen stream 191 in lieu of combining the waste nitrogen stream with the waste oxygen stream into a single waste stream as shown in FIGS. 1-4.

While the present enhancements has been described with reference to a preferred embodiment or embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present inventions as set forth in the appended claims. 

What is claimed is:
 1. An air separation unit comprising: a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; an adsorption based pre-purifier unit configured for removing water vapor, carbon dioxide, nitrous oxide, and hydrocarbons from the compressed air stream and producing a compressed and purified air stream; a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; a distillation column system having a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser-reboiler, the distillation column system further includes an argon column arrangement operatively coupled with the lower pressure column, the argon column arrangement having at least one argon column and an argon condenser, the distillation column system configured for receiving the cooled, compressed and purified air stream and produce at least two or more oxygen enriched streams from the lower pressure column; an argon product stream, a gaseous nitrogen product stream; and wherein the distillation column system further comprises a rare gas rectification system configured to receive an argon-oxygen enriched stream from the lower pressure column and to produce a crude rare gas stream.
 2. The air separation unit of claim 1, wherein at least one of the oxygen enriched streams from the lower pressure column is an oxygen product stream and at least one of the oxygen enriched streams from the lower pressure column is an oxygen enriched condensing medium directed to the argon condenser.
 3. The air separation unit of claim 1, wherein the rare gas rectification system further comprises between 2 and 6 theoretical stages of separation.
 4. The air separation unit of claim 1, wherein the rare gas rectification system is configured to recover between about 90% to 99% of the xenon and between about 10% to 90% of the krypton in the compressed and purified air stream.
 5. The air separation unit of claim 1, wherein the rare gas rectification system further comprises an additional separation section of trays disposed in the lower pressure column just above the condenser-reboiler and the crude rare gas stream is extracted from the bottom of the lower pressure column.
 6. The air separation unit of claim 1, wherein the rare gas rectification system further comprises an additional separation section of trays disposed in an argon condenser vessel just above the argon condenser and the crude rare gas stream is extracted from the bottom of the argon condenser vessel.
 7. The air separation unit of claim 1, wherein the rare gas rectification system further comprises a bifurcated separation section of trays with a first part of the separation section disposed in an argon condenser vessel just above the argon condenser and a second part of the of the separation section disposed in the lower pressure column just above the condenser-reboiler and the crude rare gas stream is extracted from the bottom of the lower pressure column.
 8. The air separation unit of claim 1, wherein the rare gas rectification system further comprises a bifurcated separation section of trays with a first part of the separation section disposed in an argon condenser vessel just above the argon condenser and a second part of the of the separation section disposed in a stand-alone rare gas column and the crude rare gas stream is extracted from the bottom of the stand-alone rare gas column.
 9. The air separation unit of claim 8, wherein the crude rare gas stream is bifurcated into a first crude rare gas stream directed to an upper location of the stand-alone rare gas column and a second crude rare gas stream directed to an intermediate location of the stand-alone rare gas column.
 10. An air separation unit comprising: a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; an adsorption based pre-purifier unit configured for removing water vapor, carbon dioxide, nitrous oxide, and hydrocarbons from the compressed air stream and producing a compressed and purified air stream; a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; a distillation column system having a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser-reboiler, the distillation column system further includes an argon column arrangement operatively coupled with the lower pressure column, the argon column arrangement having an argon column, an argon condenser, and a reboiler; wherein the distillation column system configured for receiving the cooled, compressed and purified air stream and produce a first oxygen enriched stream from the bottom of the lower pressure column; an argon product stream, a gaseous nitrogen product stream; wherein the argon column arrangement is configured to receive an argon-oxygen enriched stream from the lower pressure column at the reboiler and produce a condensed argon-oxygen enriched stream that is let down in pressure and directed to an intermediate location of the argon column; wherein the argon column arrangement is further configured to produce another oxygen enriched stream that is returned to or released into the lower pressure column and an argon-enriched overhead that is directed to the argon condenser; and wherein the argon condenser is configured to condense the argon-enriched overhead against the oxygen enriched stream taken from the bottom of the lower pressure column to produce a crude argon stream, an argon reflux stream and an oxygen enriched waste stream.
 11. The air separation unit of claim 10, wherein the argon condenser is configured to condense the argon-enriched overhead against a mixture of the oxygen enriched stream taken from the bottom of the lower pressure column and a source of liquid nitrogen to produce the crude argon stream, the argon reflux stream and the oxygen enriched waste stream.
 12. The air separation unit of claim 10, wherein the oxygen enriched waste stream is warmed in the main heat exchange system and used to regenerate the adsorption based pre-purification unit.
 13. An air separation unit comprising: a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; an adsorption based pre-purifier unit configured for removing water vapor, carbon dioxide, nitrous oxide, and hydrocarbons from the compressed air stream and producing a compressed and purified air stream; a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; a distillation column system having a higher pressure column and a lower pressure column linked in a heat transfer relationship via a condenser-reboiler, the distillation column system further includes an argon column arrangement operatively coupled with the lower pressure column, the argon column arrangement having an argon column and an argon condenser, the distillation column system configured for receiving the cooled, compressed and purified air stream and a first oxygen enriched stream from the lower pressure column; an argon product stream, a gaseous nitrogen product stream; wherein the argon column arrangement is configured to receive an argon-oxygen enriched stream from the lower pressure column and produce a second oxygen enriched stream that is returned to or released into the lower pressure column and an argon-enriched overhead that is directed to the argon condenser; wherein the distillation column system further comprises a supplemental oxygen column configured to receive another oxygen enriched stream from the argon column and rectify the received another oxygen enriched stream to produce an oxygen enriched overhead stream that is returned to the argon column and a hydrocarbon-free oxygen liquid stream; wherein the supplemental oxygen column includes a reboiler disposed proximate the bottom of the supplemental oxygen column and configured to boil oxygen in the supplemental oxygen column against a stream of nitrogen received from the higher pressure column or a portion of the compressed and purified air stream to produce an ascending oxygen vapor in the supplemental oxygen column and a condensed nitrogen stream; and wherein all or a portion of the first oxygen enriched stream from the lower pressure column is an oxygen enriched condensing medium directed to the argon condenser.
 14. The air separation unit of claim 13, wherein the another oxygen enriched stream from the argon column received by the supplemental oxygen column is a diverted portion of the second oxygen enriched stream and the oxygen enriched overhead stream is returned to the argon column.
 15. The air separation unit of claim 13, wherein the another oxygen enriched stream from the argon column received by the supplemental oxygen column is taken from an intermediate location of the argon column and the oxygen enriched overhead stream is returned to another intermediate location of the argon column just below the intermediate location of the argon column where the oxygen enriched stream is taken. 